The present invention relates to a surface light source device of side light type, and more particularly to a technique for improving quality of illuminating light in the device. The surface light source device according to the present invention is applied to, for instance, backlighting of a liquid crystal display.
It is well known that a surface light source device of side light type is applied to backlighting of a liquid crystal display. The surface light source device supplies illuminating light from the back face of a liquid crystal panel. This arrangement is suitable for making the overall shape of the display thinner.
In general, a surface light source device of side light type uses a rod-shaped light source, such as a cold cathode tube, as a primary light source. The rod-shaped light source is provided on a side of a guide plate (plate-like guide body). Illuminating light, emitted from the primary light source, is guided inside the guide plate through an incidence end face (hereinafter xe2x80x9cincidence surfacexe2x80x9d) of the guide plate. Having been guided inside, the illuminating light is propagated in the guide plate, whereby illuminating light output is obtained from a major surface of the guide plate.
A plate of generally uniform thickness and a plate of gradient thickness are known as types of plate which can be used as the guide plate. In general, the latter has higher illuminating light output efficiency than the former.
FIG. 8 is an exploded perspective view of a surface light source device of side light type using the latter type of guide plate. FIG. 9 is a cross-sectional view taken along the line Axe2x80x94A of FIG. 8. As shown in FIG. 8 and FIG. 9, the surface light source device of side light type 1 comprises a guide plate 2, a primary light source 3 which is provided along one side of the guide plate 2, a reflection sheet 4, a prism sheet 5 and a diffusion sheet 6.
The reflection sheet 4, the guide plate 2, the prism sheet 5 and the diffusion sheet 6 are laminatedly arranged. The guide plate 2 is a plate-like guide member which is bar-shaped in cross-section. In this example, the guide plate 2 comprises a scattering guide body. The scattering guide body comprises, for instance, a matrix of PMMA (polymethyl-methacrylate) and a great number of light-permeable particles which are uniformly dispersed therein. The refractive index of these particles is different from that of the matrix. Such a guide plate is called a scattering guide plate.
The guide plate (light-scattering guide plate) 2 has major surfaces providing an emission surface 2C and a back face 2B. The emission surface 2C provides a prism surface as a light-controlling surface. This prism surface comprises a great number of rows of projections. As shown in partial enlargement at the section indicated by reference symbol B, the projections, each having slopes 2E and 2F, run almost perpendicular to the end surface (incidence surface) 2A and are triangular in cross-section. As is well known, this type of prism surface functions by gathering light propagation direction toward the frontal direction in a surface parallel to the incidence surface 2A.
A guide plate comprising transparent acrylic resin may, for instance, be used instead of the scattering guide plate 2. When a transparent guide plate is used, a diffusion surface is conventionally provided on the back face 2B.
The primary light source 3 comprises a bar-shaped cold cathode tube (flourescent lamp) 7 and a reflector 8, generally semicircular in cross-section, which is provided to the back face of the cold cathode tube 7. Illuminating light is supplied through the opening of the reflector 8 toward the side end face of the guide plate 2. A sheet-like regular reflection member comprising metal foil or the like, or a sheet-like diffused reflection member comprising white PET film or the like, is used as the reflection sheet 4.
The prism sheet 5 is a light-permeable sheet-like member comprising, for instance, polycarbonate. The prism sheet 5 is normally provided with the prism surface facing the scattering guide plate 2.
Each prism surface comprises a great number of projections which run parallel to each other. As shown in partial enlargement in the section indicated by reference symbol C, each projection has a slope 5A and a slope 5B. Then, the prism sheet 5 is aligned so that these projections run almost parallel to the end surface (incidence surface) 2A. As is well known, the prism sheet 5 aligned in this manner corrects the direction of light propagation to the frontal direction in a surface which is perpendicular to the incidence surface 2A.
The diffusion sheet 6, provided on the outer side of the prism sheet 5, diffuses light propagation direction. In general, the diffusion power of the diffusion sheet 6 is weak and illuminating light is scattered weakly in order to prevent interference fringes from occurring.
Illuminating light L from the primary light source 3 is led through the incidence surface 2A into the guide plate 2. Inside the guide plate 2, the illuminating light L propagates toward the end while being repeatedly reflected between the back face 2B and the emission surface 2C. During this process, the illuminating light L is subjected to scattering action of the particles inside the guide plate 2. A portion of light leaks from the back face, but the mechanism of sheet 4 effectively returns this leaked light into the guide plate 2. When the reflection sheet 4 comprises a diffused reflection member, diffused reflection action also takes effect.
As can be understood from FIG. 9, since the back face 2B is inclined with respect to the emission surface 2C, the angle of incidence of the illuminating light to the emission surface 2C gradually decreases with each reflection of illuminating light L from the slope 2B. This reduction in the angle of incidence increases the incident components which are below the critical angle to the emission surface 2C. This facilitates emission from the emission surface 2C as the illuminating light nears the end. Consequently, reduced brightness in regions which are far from the primary light source 3 is prevented.
The illuminating light L output from the emission surface 2C has properties of scattered light because it has experienced scattering by light-permeable particles, or further diffused reflection by the reflection sheet 4. However, as is well known, the priority propagation direction (the main direction of propagation) inclines in the end direction (opposite direction to the primary light source 3) with respect to the frontal direction. The prism sheet 5 corrects such directivity and corrects the priority propagation direction to the frontal direction in a surface perpendicular to the incidence surface 2A. The diffusion sheet 6 weakly scatters the illuminating light, eliminating cause of minute brightness inconsistencies such as interference fringes.
In general, such a surface light source device 1 using the bar-shaped guide plate 2 and the prism sheet 5 emits light in the frontal direction more efficiently than a surface light source device of the same type using a guide plate of generally even thickness.
However, in the conventional device described above, undesirable bright lines are generated along both side edges on the emission surface 2C (left and right belt regions as viewed from the primary light source 3). These bright lines tend to be especially noticeable when the emission surface 2C comprises a light control surface (a great number of projections) as described above. The bright lines comprise localized fine belts of high brightness, reducing the evenness of light output.
Such a tendency of bright lines might be lessened by using a flat emission surface having no projection, but further restriction is desirable. Furthermore, the guide plate 2 would lose its function of correcting directivity in a surface parallel to the incidence surface 2A. Cause of bright lines, which occur along both side edges on the emission surface 2C, is thought to be that illuminating light illuminates these side edges, undergoes one or more internal reflections and is emitted locally from the emission surface 2C.
The projections on the emission surface 2C relax the critical angle conditions for light escape and consequently facilitate local emission. Such a phenomenon is known as xe2x80x9c(side edge) over-reflectionxe2x80x9d. There is a demand to remove such side edge over-reflection in order to improve the quality of light output in a surface light source device of side light type.
The present invention aims to solve the abovementioned problems of the conventional surface light source device of side light type. It is an object of the present invention to prevent generation of bright lines along side edges of the emission surface in a surface light source device of side light type. Described from another point of view, the present invention aims to enable generation of bright lines along side edges of the emission surface to be prevented even when the surface light source device of said type employs a guide plate having an emission surface which comprises a light control surface.
The present invention is applied to a surface light source device of side light type comprising a primary light source and a guide plate, having an emission surface and a back face as major surfaces thereof, wherein light is supplied from an incidence end surface of the guide plate, and an edge is formed by a side face, adjoining the incidence end surface, meeting with a back face.
In compliance with the features of the present invention, multiple projections are provided on the back face in a region along the edge. These projections run generally parallel to the edge. A great number of projections, running generally perpendicular to the incidence end surface, may be provided on the emission surface.
In compliance with the preferred embodiment, projections provided on the back face decrease in height as their distance from the edge increases. Furthermore, in compliance with another preferred embodiment, the projections, provided on the back face, decrease in sharpness as their distance from the edge increases.
The projections, provided on the back face of the guide plate in a region along the edge, diversify and spread the propagation direction of illuminating light, arriving from the emission surface of the guide plate and the side face edge, and light arriving from a frame, which forms a peripheral member. Consequently, light arising from such illumination of the edge or the frame is prevented from being strongly emitted locally from the emission surface thereafter. As a result, generation of bright lines along the side edges of the emission surface is prevented. Bright lines of this type are especially liable to occur when a prism surface is provided on the emission surface, but they can be effectively controlled by the application of the present invention.